A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Spectroscopic scatterometry has the benefit of the hardware is relatively simple, which helps to improve matching and calibration. However, it has difficulties measuring very isolated features, and asymmetry of the metrology gratings. Angle-resolved scatterometry is optically more complicated, which complicates calibration and matching. Moreover, in practice, multiple adjustable wavelengths are needed, which leads to complex and expensive optics. As the resolution of lithographic processes increases, ever smaller features are created on substrates. In order to perform scatterometry at the resolution of the smallest features, it may be desirable to use shorter wavelengths of radiation, comparable to those used in the lithographic process itself. Wavelengths in the ultraviolet (UV) range may be effective for this in principle. However, optical systems for such wavelengths become particularly complex.
There is accordingly a desire for new forms of scatterometer, particularly ones suitable for measuring metrology targets with feature sizes at the resolution of current and next-generation lithographic processes. The inventor has recognized that a limitation of known spectroscopic scatterometers is that they make no use of higher diffracted radiation from the target grating.
A new form of scatterometer has been proposed in the paper “A New Approach to Pattern Metrology” Christopher P. Ausschnitt, published in Metrology, Inspection, and Process Control for Microlithography XVIII, edited by Richard M. Silver, Proceedings of SPIE Vol. 5375 (SPIE, Bellingham, Wash., 2004), DOI: 10.1117/12.539143. Unlike conventional spectroscopic scatterometers, Ausschnitt's so-called MOXIE system uses both zero order and first order diffracted radiation. It also uses a target grating on the substrate itself to resolve the diffracted orders into a spectroscopic signal. However, this system is also not optimized for measuring pattern asymmetries. Moreover, the spectral resolution of the first order signal is dependent on the target geometry and is expected to be too small for practical metrology applications.
Another problem in known scatterometry techniques is the space or “real estate” occupied by scatterometry targets on product substrates. Targets must be kept away from one another and from product features, to avoid cross-talk between measurements. The inventor has further recognized that one cause of cross-talk is that an illumination spot of the instrument has a point spread function with significant sidelobes of energy around a main spot.
A problem in lithographic processes generally is that height measurements used for controlling the transfer of a pattern to a substrate may be influenced unpredictably by process-dependent influences.